1. Filed of the Invention
The present invention relates to an electrophotographic process and more particularly to an improvement of an electrophotographic copier and charging means used therefor.
2. Description of the Related Art
As is well known, there have been many proposals for duplicating machines and particularly, in recent years, most of these are proposed electrophotographic copiers. Such an electrophotographic copier is typically, elementally constructed of a photoconductor drum, a charging unit, an exposure unit, a developing unit, an image transfer unit, an erasing unit and a cleaner, and all the elements are disposed around the photoconductor drum to effect a series of electrophotographic process. In addition, there are arranged elementally a paper feed tray, paper guides, paper feed rollers, the image transfer charging unit, a suction unit (for conveying), a fixing unit and paper discharge rollers. With such configurations, an image transferred on a sheet is fixed to create a duplication. More specifically, as shown in FIG. 1 of a schematic view, an image forming apparatus based on electrophotography comprises a photoconductor drum 1, in which a photoconductive film is formed on a conductive support, and a series of the following elements disposed upstream to downstream of a rotational direction of the photoconductor drum 1, that is, a charging unit 102, an exposure unit 103 for illuminating light on the photoconductor drum 1 impressed at a charging potential by charging unit 102 to discharge the static charges on the photoconductor drum 1 and create a desired electrostatic latent image, a developing unit 104 for supplying toner powder to the photoconductor drum 1 having the electrostatic latent image, an image transfer unit 106 for transferring the toner powder image on the photoconductor drum 1 onto a recording sheet 105, a fixing unit 107 for melt-fixing the tonered image transferred on the recording sheet 105 by heating and/or pressing, an erasing unit 108 for erasing the static charges remaining on the photoconductor drum 1 after light-irradiation on the photoconductor drum 1 and image transfer, and a cleaner 109 for removing the residual toner on the photoconductor drum 1.
Of these, as the charging unit for charging the photoconductor at a desired potential a corona charger utilizing a corona discharge phenomenon has been used in the prior art. This means requires a high voltage, so that there has been a fear that the voltage may influence a microcomputer, etc. To make the matter worse, upon the corona-discharging, a large quantity of ozone gas will be generated which not only deteriorates resin material used for the cleaning blade, etc, but also gives unpleasant feelings, causing environmental problems. To eliminate such problems, charging means which charges a photoconductor by an electro-conductive roller or fiber aggregation applied with a voltage has been proposed, for example, in Japanese Patent Application Laid-Open sho-55 No.29837.
FIG. 2 shows an oblique view of an example of such a prior art charging means. In the figure, reference numeral 1 designates a photoconductor drum, of which surface 1a is in contact with conductive fiber 5a planted brush-wise on a fiber substrate 5d made of aluminum or other conductive material.
In this case, since the mechanism is constructed such that the conductive fiber 5a fixed is brought into contact with the photoconductor surface 1a, the structure might be simple, but the developer and other foreign substances easily to build up between fibers or tips of fibers, causing abnormal discharge resulting in a reduction of the fiber life, and/or causing changing unevenness.
On the other hand, in order to improve the situation, there is disclosed a charging device which, obliquely shown in FIG. 3, comprises, for example, a shaft 5c and conductive fiber 5a (as stated above) planted therearound to form a roll-shaped member. This roll-shaped member is rotated relative to the photoconductor drum 1 by a driver (not shown). As a result, the reduction of the fiber life and changing unevenness which are caused by the adhesion of foreign substances or other reason can be remedied and bettered remarkably.
Another example of prior art is shown perspectively in FIG. 4, in which there are provided a photoconductor drum 1, a photoconductive medium 1a made of a photoconductive dielectric layer, a charging member 5 comprising a roller shaft 5c covered with conductive rubber therearound. As shown in the figure, the charging mechanism of this kind has typically utilized elastic rollers as its changing means. In other words, a substance used for the member had to have a highly smooth surface and to be less changed or degraded with the passage of time, in order to afford uniform discharge. In addition, the means was required to be constructed such that, the charge supplying member should be prevented from damaging and the charge supplying member should not be voltage-dropped totally, the in case where an abnormal current arose through the charge supplying member due to pinholes on the photoconductor, or other cause.
Accordingly, in order to provide a charging member as described above, Japanese Patent Laid-Open hei-2 No. 62563 discloses use of a charging brush that is planted with the fibers looped substantially perpendicular to a rotational direction of the image bearing medium (photoconductor) formed on the photoconductor drum surface.
FIG. 5 is an illustration showing the structure, and there are disposed photoconductor drum 1 with an image bearing medium 1a (photoconductor). Reference numeral 5 designates a charging member having charging brush which is formed with conductive fibers 5a looped shown in the figure. The looped fibers 5a are planted on a conductive substrate 5d with a 5g conductive adhesive to thereby form charging brush 5. In this case, the photoconductor drum 1 rotates in a direction shown by arrow R, while the conductive fibers 5a are planted so that the loop structure is perpendicular to the moving direction of the photoconductor drum surface.
Using this means could reportedly inhibit stripe-like charging unevenness from occurring, compared to the conventional charging brush.
Meanwhile, charging members using such conductive fiber can be conceivably classified into two kinds, one of which is constructed as shown in FIGS. 2 and 5 such that a charging member is formed like a brush and fixed stationary in sliding contact with the surface of the photoconductive material 1a. The other type of the charging members is formed as a roll and the roll-shape member is brought into contact with photoconductive material 1a relatively with moving on the surface of photoconductive material 1a. The former one has a simple structure but exhibits a tendency that the fiber is built up with toner or other foreign substances, still likely causing charging unevenness. In the latter case, since the conductive fiber aggregation 5a moves, foreign substances is hard to build up, and an additional cleaning means might also be provided. Nevertheless, the structure becomes complicated, and when for example, the conductive fiber cloth is wound roll-shaped or belt-wise, the seam formed may cause charging unevenness.
Causes of thus occurring charging unevenness were studied, and the following views were realized.
First, it is generally known that the surface of photoconductor 1a will be charged when photoconductor 1a is brought into contact with conductive fiber aggregation 5a to which a voltage is applied. This electrification is conceivably caused both by discharge across the micro-clearance and by charge-injection from the contact points. The discharge across the micro-clearance starts to occur when the voltage across the clearance reaches a certain level. This voltage is determined by Paschen's rule of discharge, and an example of the relation is shown in FIG. 6. Once the discharge occurs, charges transfer all at once from conductive fiber aggregation 5a to photoconductor 1a. This transfer causes the surface potential of photoconductor 1a to heighten and then the discharge stops. Even after completion of the discharge, photoconductor 1a is still elevated in its surface potential by the injection of charges from the contacts points. For this reason, a portion which comes in touch with conductive fiber aggregation 5a in a longer time, or a portion which contacts thereto at a higher possibility will bear higher potentials. This can be realized as to be the cause of charging distribution unevenness appearing in broomed traces or seams of conductive fiber aggregation 5a.
On the other hand, charging unevenness of the stripe-type generated in the brush-type charger is mainly attributed to long-term contact of the brush-like charging member made up of conductive fiber against the same contacting point on the image bearing medium. In addition, such a contact over a long period of time not only rubs certain points on the image bearing medium repeatedly causing possible scratches and wounds on the medium, but also wears the brush itself quickly. To make the matter worse, the developer may gradually be built up in the tips of the brush resulting in pollution.
The adhesion of the developer to the ends of nap or fibers of the conductive fiber in the charger may deteriorate the fiber itself in its durability. Further, a long term contact of the charger onto the surface of the image bearing medium brings down the conductive fibers in a rotating direction of the medium, and the thus worn-out fiber cannot allow itself to keep uniform contact with the surface of the image bearing medium, causing non-uniformity of charging to generate charge-distribution unevenness.
Still, fibers are generally liable to absorb moisture, and fibers with dampness become too flexible, making it difficult for the fibers to stand upright. For this reason, once the fiber is exposed in a high humidity environment, the worn-out, or the state of being brought down of, the fiber cannot be cured.
Meanwhile, used as a photoconductive material for the photoconductor drum are organic semiconductors, CdS, SeTe, As.sub.3 Se.sub.2, etc, of which organic semiconductors are mostly used. Typically, an N-type organic semiconductor bearing negative charges presents good attenuation characteristics in response to light exposure, but the same semiconductor bearing positive charges exhibits poor light-attenuation characteristics.
For this reason, when a positive transfer voltage, that is, the same polarity with the charging voltage, is applied by the transfer roller to the surface of the photoconductor drum even through a recording sheet therebetween and thereafter the recording sheet is separated from the photoconductor drum, the potential of the photoconductor drum surface to which the transfer voltage is not applied receives some influence. Accordingly, when the charging voltage is applied by the roller to the photoconductor drum after the transfer, a difference due to the aforementioned transfer voltage is caused to appear in the surface potential of the drum, by the electrificability of the charge roller. The difference in the surface potential has influence on image, causing fog and density irregularity in the final image.
There have been several proposals other than the above that use such charging means of contact type.
For example, Japanese Patent Application Laid-Open sho-59 No. 204859 discloses a means for preventing deterioration due to wear-out of a brush for use in a brush roller, planted with conductive fibers thereon as charging means, and contacted against a photoconductor. This mechanism is provided with a cum and a tracking roll in each end of the photoconductor and in each end of the brush roller, respectively, and the tracking rolls run on the cum surfaces and the tracking rolls step on respective projections disposed on the cums when the copier is out of operation, whereby the front ends of the brush are kept spaced from the surface of the photoconductor. However, such a structure not only increases the number of parts for the copier, but also requires control of the tracking rolls to step on the projections, and consequently the means cannot be realized as being very practical.
Another publication in Japanese Patent Application Laid-Open sho-60 No. 216361 discloses a means serving as both charging means and transfer means, comprising a roller or brush planted with conductive fibers to be brought in contact with a photoconductor, the means in which a first cycle performs a charging operation while a second cycle effects a transfer operation. In this case, a conductive member is applied by a combined voltage of a d.c. voltage and an a.c voltage of 20% or more of the d.c. voltage, where maximum and minimum values of voltage waveform for the a.c.-overlapped d.c. voltage are to be within .+-.200 to .+-.2000 volts. This measure requires no switching of the applied voltages between a charging operation and a transfer operation, and improves uniformity of charging as well as achieves an excellent transfer efficiency. However, since this means is oriented to suffice a special usage for effecting both charging and transfer operations, the structure tends to be complicated.
Another disclosure in Japanese Patent Application Laid-Open sho-64 No. 73367 shows a charging means constructed such that, in charging a photoconductor by bringing a contact-type charging member, such as a conductive roll, which is applied with a combined voltage of d.c. and a.c. voltages, into contact with the photoconductor, a portion by which the charging member is in contact with the photoconductor is formed with a resistance layer and a dielectric layer as a surface layer, and therefore a reactance of the charging member to a.c. voltage is smaller than the resistance of the charging means. Use of this means may prevent voltage-drop of the voltage supplying portion due to a leak even though pin-holes may happen to occur on the surface of the photoconductor, and thus the image unevenness that would be caused by the voltage drop will not occur. Here, according to this publication, it is described that the frequency of the a.c. voltage used should be within a range of 50 to 2000 Hz. This proposal was made mainly to eliminate the lowering of image quality attributed to wounds such as pin-holes and other defects arising on the surface of the photoconductor, therefore, the concept on which the technology is based is quite different from what the present invention intends to achieve.